1. Field of the Invention
The present invention relates to a magnetooptic sensor head that uses the Faraday effect of a bismuth-substituted iron garnet, and more particularly to the construction of a magnetooptic sensor head which senses the intensity of a magnetic field and is inexpensive, small in size, light weight, and suitable for mass production.
2. Prior Art
Today, many conventional industrial apparatuses and consumer equipment include rotating devices or rotating mechanism such as motors and gears. The advances in science and technology have enabled industry to accurately control apparatuses for industrial use such as aircraft and ships and consumer apparatuses such as automotive vehicles in order to address the problems such as conservation of global environment and energy saving. In order to implement higher level and more accurate control of rotating apparatuses, the rotational speeds thereof must be measured both continuously and accurately. This measurement requires accurate measuring devices which are simple, light weight, and readily available at low costs and in large quantity. The measurement of rotational speeds includes a variety of methods. For example, a rotational speed meter as shown in FIG. 1, which makes use of electromagnetic induction, has been developed and used for measuring the rotational speeds of engines for aircraft and automotive vehicles (Sensor Gijutsu, page 68, December, 1986). Another way of measuring rotational speeds has been proposed which makes use of a magnetooptic sensor head based on the Faraday effect of magnetooptic materials (Applied Optics, Vol.28, No.11, page 1,992, 1989).
The probe of a rotational speed meter for the engines of aircraft and automotive vehicles are based on electromagnetic induction. This type of rotational speed meter is susceptible to electromagnetic noise that comes in through the cables connecting the probe and the main body of the meter. Furthermore, since electric circuits are involved, this type of rotational speed meter must be designed so that the electric circuits will not cause explosion when used in an environment where flammable materials such as organic solvents are used or stored.
A magnetooptic sensor head based on the Faraday effect of a magnetooptic material makes use of a the rotation of polarization plane of the material in response to the presence and absence of a magnetic field (or magnet) when a permanent magnet (or magnetic field) approaches the magnetooptic material. That is, the rotation of the polarization plane of light that is transmitted through a magnetooptic material incorporated in a magnetooptic sensor head, is converted into changes in light intensity, and the number of changes is counted to determine the rotational speed (National Technical Report, Vol.29, No.5, p70, (1983)).
FIG. 1 shows general construction of a transmission type magnetooptic sensor head. In FIG. 1, light emitted from light source 1 such as a semiconductor laser, enters a polarizer 2 in the form of, for example, calcite. The light exists the polarizer 2 as a linearly polarized light where the polarization plane is in one direction, and enters a Faraday rotator 3 which is usually made of a magnetooptic material such as zinc selenide (ZnSe).
The polarization plane of the light exiting the Faraday rotator 3 has been rotated through an angle .THETA..sub.F in accordance with an applied external magnetic field Hex. The light exiting the Faraday rotator 3 then enters an analyzer 4 made of, for example, calcite.
The intensity P of the light from the analyzer 4 is given by EQU P=k cos.sup.2 (o-.THETA..sub.F) (1)
where o is an angle of the polarizer 2 relative to the analyzer 4, .THETA..sub.F is an angle through which the polarization plane rotates, and k is a proportional constant. Assuming that o is equal to 45 degrees, Equation (1) can be rewritten as follows: EQU P=k (1+sin2.THETA..sub.F)/2 (2)
Further, if .THETA..sub.F is sufficiently small, Equation (2) can be approximated as follows: EQU P=k (1+2.THETA..sub.F)/2 (3)
Equation (3) indicates that the intensity of light becomes proportional to .THETA..sub.F if o is selected to be 45 degrees. In other words, the rotational angle .THETA..sub.F of the polarization due to applied external magnetic field Hex can be detected or measured in terms of the intensity of light by the use of the analyzer 4.
Various proposals have been made for the systems and configurations of a magnetic field sensor head. They can be grouped into transmission type and reflection type. With the type of sensor head shown in FIG. 1, because of the nature of the structural elements, the elements must be aligned in a straight line so that the light travels straight. Thus, if obstructions are located in the light path, the magnetooptic sensor head cannot be placed properly.
Japanese Patent Preliminary Publication No.56-55811 discloses a reflection type magnetooptic sensor head which overcomes the deficiencies of the transmission type. FIG. 2 shows the general construction of a reflection type magnetooptic sensor head. In FIG. 2, a signal light passes through an optical fiber 5a and then through a lens 6a to a polarizer 7 made of, for example, rutile single crystal. The light from the polarizer 7 enters a Faraday rotator 8 to a rectangular prism 9 which reflects the light back to the Faraday rotator 8. The light from the Faraday rotator 8 is then incident upon the analyzer 10 and is then coupled via a lens 6b to an optical fiber 5b.
The construction of the reflection type in FIG. 2 differs from the transmission type in that the rectangular prism 9 is provided to reflect the signal light. With a reflection type magnetooptic sensor head shown in FIG. 2, the input optical cable is aligned in parallel with the output optical cable. In other words, the Faraday rotator 8 is disposed at the tip end portion of the sensor head. Thus, a reflection type magnetooptic sensor head is advantageous in that the sensor head can be installed in a narrow space where a transmission type magnetooptic sensor head cannot be installed. However, the reflection type sensor head of Matsui et al. is disadvantageous in that the lens 6a must be in series with the polarizer 7, the lens 6b must be in series with the analyzer 10, and these two series connections must be in parallel with each other. This requirement of aligning the elements places limitations on the automatic assembly operation of the entire system, and is not cost effective.
FIG. 3A relates to Japanese Patent Publication No.3-22595 (Natsumura et al.) which proposes a configuration where the polarizer 7 and the analyzer 10 are replaced by a single polarizer. This configuration overcomes the deficiency of the reflection type magnetooptic sensor head proposed by Matsui et el.
In FIG. 3A, the light emitted from a light source 11 such as a semiconductor laser, passes through a lens 12 and a half mirror 13. The light then enters an optical fiber 14. The half mirror 13 permits part of the light to pass through and reflects part of the light incident thereupon so that the reflected light enters a light path 1. A photodetector or power meter 18 placed in the light path 1 and serves to measure variations in the intensity of light emitted from the light source 11. The signal light directed to the optical fiber 14 passes through a polarizer 15 made of, for example, futile single crystal, and a Faraday rotator 16 made of magnetooptic material to a reflecting film 17. The reflecting film 17 is usually made of a metallic thin film.
The light that reached the reflecting film 17 is then reflected back to the Faraday rotator 18 and then to the polarizer 15. The returning light through the polarizer 15 enters the optical fiber 14. The returning light exiting the optical fiber 14 enters the half mirror 13 which reflects in part the light into a light path 2. The light passing through the light path 2 then enters the photodetector 19 which measures the intensity of the light.
Matsumura et al. employed yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12), usually referred to as YIG, as a Faraday rotator produced by a flux melt technique. YIG is advantageous as a Faraday rotator element in that the Faraday rotation coefficient (deg/cm) is larger in YIG than in paramagnetic glass and zinc selenide. The use of YIG proposed by Natsumura et al. is one way of overcoming the deficiency of a reflection type magnetooptic sensor head proposed by Natsui et al.
In fact, the use of YIG is attractive in terms of ease of production. However, YIG may not be practical as a Faraday rotator since it is well known that YIG transmits light in the near infrared range having wavelengths longer than 1.1 .mu.m and absorbs light in the 0.8 .mu.m band.
Conventionally, an optical sensor head uses a light source such as a semiconductor laser (LD) or light emitting diodes (LED) having median wavelengths in the range of 0.78-0.85 .mu.m. Semiconductor laser and light emitting diodes are used as a light source for a photosensor because they are very inexpensive in the above wavelength range, and because photodetectors have good sensitivity in that range. Thus, using light sources available on the market is most preferred and is the best way to provide inexpensive magnetooptic sensor heads that meet the user's needs.
High light absorption of YIG implies that the detection of light may be difficult if a conventional light source is used. That is, YIG is inherently deficient as a Faraday rotator.
The inventors of the present invention investigated many other materials in order to overcome the deficiency of YIG. The inventors concluded that bismuth-substituted iron garnets can be used as a magnetooptic material. The bismuth-substituted iron garnets can be manufactured rather easily by the LPE (Liquid Phase Epitaxial) method, and lends itself to mass production. Bismuth-substituted iron garnets are represented by a chemical formula (RBi).sub.3 (FeA).sub.5 O.sub.12, where R represents yttrium Y or rare earth elements and A represents aluminum Al and gallium Ga.
The Faraday rotation coefficient of a bismuth-substituted iron garnet, i.e., the rotation angle of the polarization plane per unit film thickness at the saturated magnetization is as large as several times as large that of YIG, about ten times as large at the 0.8 .mu.m band. This indicates that the film thickness can be smaller with increasing Faraday rotation coefficient for the same magnetooptic effect, achieving less light absorption loss and smaller size. The film thickness of an element can be smaller in bismuth-substituted iron garnets than in YIG, indicating less light absorption. Thus, bismuth-substituted iron garnets are useful in implementing a magnetooptic sensor head with a light source having a wavelength in the 0.8 .mu.m band.
The magnetic saturation of bismuth-substituted iron garnets ranges from 500 to 1200 Oe which are about half that of YIG (about 1800 Oe). This indicates that the bismuth-substituted iron garnets can be used to measure weak magnetic fields as well. The ability to measure weak magnetic fields implies that the distance between the permanent magnet and the magnetooptic sensor head can be longer. This provides more flexibility and higher degrees of freedom in installing the magnetooptic sensor head and suggests wider fields of application for magnetooptic sensor heads.
With the aforementioned investigation, the inventors of the present invention believed that reflection type magnetooptic sensor heads can be developed by the use of a bismuth-substituted iron garnet as a Faraday rotator. Based on the disclosure in Japanese Patent Publication No.3-22595, the inventors of the present invention built an engineering model of a reflection type magnetooptic sensor head as shown in FIG. 3B using a Faraday rotator made of a bismuth-substituted iron garnet single crystal in place of YIG.
In FIG. 3B, the light emitted from a light source 51 such as a semiconductor laser, passes through a lens 52 which condenses the light, and then through a half mirror 53 which reflects the light in part and transmits the rest of the light. The light then enters an optical fiber 54. The light directed to the optical fiber 54 is converted by a lens 55 in the form of, for example, a gradient-index rod lens into a parallel light before entering the polarizer 56 which converts the light into a linearly polarized light. The linearly polarized light enters a Faraday rotator 57 made of a (111) bismuth-substituted iron garnet and part of the light is transmitted through the Faraday rotator 57 to a reflecting film 58 made of, for example, a metallic thin film. The light impinging on the reflecting film 58 is then reflected back and enters the Faraday rotator 57 and then the polarizer 56. The light exiting the polarizer 56 is then coupled to the optical fiber 54 via the lens 55. The light passes through the optical fiber 54 into the half mirror 55 which reflects the light in part to a photodetector 59 or power meter which determines the light intensity.
Using the thus built reflection type magnetooptic sensor head, the inventors made a variety of experiments for various magnetic field intensities. However, the sensor head failed to detect any light signals regardless of whether a magnetic field was applied.
Therefore, the inventors made various experiments in order find out why the reflection type magnetooptic sensor head according to FIG. 3B failed to detect the light signals. Having made many experiments, the inventors finally realized that the sensor head failed to detect light due to the magnetic domain structure of the Faraday rotator. The inventors realized that a reflection type magnetooptic sensor head of the construction in FIG. 3B cannot detect light signals if the Faraday rotator is made of a multidomain element such as bismuth-substituted iron garnets, which have a number of magnetic domains.
The result obtained by the inventors of the present invention do not agree with the experimental results obtained by Matsumura et al. who used YIG as a Faraday rotator which also has multidomain structure as in a bismuth-substituted iron garnet. The inventors wondered why a bismuth-substituted iron garnet with multidomain did not properly function as a Faraday rotator in the reflection type magnetooptic sensor head built according to the construction in FIG. 3B similar to the YIG in Japanese Patent Publication No. 5-22595, while a YIG having the same multidomain functioned properly as a Faraday rotator in Japanese Patent Publication No. 3-22595.
Having reviewed the aforementioned experimental results and making further basic experiments, the inventors confirmed that a reflection type magnetooptic sensor head can be constructed of a reflecting film, (111) bismuth-substituted iron garnet single crystal, polarizer, and light-inputting/outputting paths. Further, the light-inputting/outputting paths are divided into two light paths; an incoming-light path for the light coming into the polarizer from a light source and an outgoing light path for the light leaving the polarizer back to the light source. The two light paths are aligned such that they make an angle greater than five degrees with respect to each other. The inventors further continued the research work of magnetooptic sensor heads and developed a reflection type magnetooptic sensor head using a Faraday rotator made of a bismuth-substituted iron garnet as disclosed in Japanese Patent Application No.4-90976.
FIG. 4 shows the construction of a reflection type magnetooptic sensor head disclosed in Japanese Patent Application No. 4-90976. In FIG. 4, a polarizer 20 is in the form of, for example, POLARCORE (Trade name) sold by CORNING and a Faraday rotator 21 is made of a (111) bismuth-substituted iron garnet single crystal film which is magnetized most easily in a direction normal to the film surface. The Faraday rotator 21 is exposed to a magnetic field to be measured. A reflecting film 22 is in the form of, for example, the multilayer of a dielectric material. An optical waveguide 25 for incoming light is formed on glass or polymer, or is in the form of an optical fiber optical waveguide 24 for outgoing lights is formed on glass or polymer, or is in the form of an optical fiber. The light emitted from a light source 26 such as semiconductor laser, is directed through a lens 25 into the incoming light path 25. The light exiting the light path 25 then passes through the polarizer 20, the Faraday rotator 21 to the reflecting film 22. The light is then reflected by the reflecting film 22 back through the Faraday rotator 21, the polarizer 20, the light path 24 to a photodetector 27 which detects the light as a light signal. With the reflection type magnetooptic sensor head in FIG. 4, the light inputting/outputting port has two independent paths 23 and 24 which make an angle .alpha. greater than five degrees relative to each other.
The aforementioned reflection type magnetooptic sensor head using a Faraday rotator made of a bismuth-substituted iron garnet, meets the requirements for a magnetooptic sensor head. However, as shown in FIG. 4, the two light paths must be aligned such that they make an angle .alpha. greater than five degrees with respect to each other. This construction is disadvantageous in implementing a sensor probe having a diameter less than five millimeters. Thus, the sensor is no use for measuring a magnetic field in a very narrow space such as a cylinder provided in the rotating shafts of gyros or turbines where the diameters are on the order of several millimeters. Further, the sensor of FIG. 4 has the disadvantage that the cost of optical fibers may become a problem if the distance between the probe (magnetooptic sensor head) and the photodetector (magnetic field measuring device) is long.